Photocathode devices are optoelectronic detectors which use the photoemissive effect to detect light energy. Thus, when photons impinge the surface of a photocathode device, the impinging photons cause electrons to be emitted therefrom. Many photocathode devices are made from semiconductor materials such as gallium arsenide (GaAs) which exhibit the photoemissive effect. While GaAs is preferred, it is noted that other III-V materials can be used such as GaP, GaInAsP, InAsP and so on. In a semiconductor photocathode device, photons are absorbed by a photoemissive semiconductor material. The absorbed photons cause the carrier density of the semiconductor material to increase, thereby causing the material to generate a photocurrent.
Semiconductor photocathode structures are employed in the image intensifiers of state of the art night vision devices. These photocathode structures typically use a semiconductor epilayer for the photon absorbing material. The semiconductor epilayer is thermally and mechanically bonded to a glass face plate of the image intensifier to provide a rigid, vacuum supporting tube structure. The peripheral surface of both the semiconductor epilayer and the glass face plate are coated with a conducting material to provide an electrical contact to the photocathode semiconductor structure. No additional contacts to the photocathode structure are provided. The image intensifier generates a photocurrent when photons are absorbed by the semiconductor epilayer, which results in the generation of carriers within the bulk of the epilayer. The carriers, without the influence of an electrostatic field applied at the contact, diffuse to the emission surface of the epilayer where a threshold energy or barrier exists. The carriers are emitted into the vacuum within the tube structure if the energy of the carriers is sufficient to either surmount the barrier or tunnel through the barrier at the emission surface. The carriers which make it through the barrier are emitted by the semiconductor photocathode and are accelerated and/or amplified by subsequent tube components to generate an intensified output image.
In order for the carriers to maintain a high enough energy to surmount or tunnel through the barrier as they diffuse to the emission surface, many present semiconductor photocathode designs utilize heavily doped semiconductor materials. The heavy doping is made uniform over the entire active depth of the photocathode. Other present semiconductor photocathode designs use a heavily doped semiconductor region near the glass plate and a lower doped region near the emission surface. The heavy doping in both designs limit the distance over which conduction band bending takes place since the lower doped concentration of the high/low doped design is essentially the same as the doping concentration used in the uniform heavily doped design described earlier. Conduction band bending over a long distance leads to the carriers losing energy and not being emitted to the vacuum. More specifically, the use of a highly doped semiconductor material in present photocathode designs provide a very small space charge region to excite the carriers over a very small distance. The heavy doping also severely limits carrier diffusion lengths, therefore, limiting the cathode thickness which ultimately limits the amount of photons which can be absorb by the semiconductor material, which in turn, limits the photogeneration capabilities of the photocathode structure.
The uniformly high or high/low doping of the semiconductor material in conjunction with the single contact to the photocathode structure, does not allow the carriers to be excited which would substantially improve the probability for emission into the vacuum.
Accordingly, there is a need for a semiconductor photocathode structure that substantially overcomes the problem of carrier emission.